1. Field of the Invention
The present invention is generally related to a method and apparatus for improving a user""s ability to gain access to a communication system and in particular to a method and apparatus for adjusting adaptively the power of a user equipment probing signal to increase the likelihood of detection by the communication system equipment.
2. Description of the Related Art
Communication systems, and in particular, wireless communication systems comprise a plurality of communication channels. through which subscribers of such systems communicate with each other and with the system. A portion of a typical wireless communication system is shown in FIG. 1. The wireless communication system of FIG. 1 depicts a cellular system comprising cells (e.g., 102, 104, 106, 108) whereby each cell is a symbolic representation of the physical terrain or geographical region covered by communication network equipment commonly known as cell sites or base stations (e.g., 110, 112, 114).
Each base station has system equipment comprising radio equipment (i.e., transmitter, receiver, modulator, demodulator) that are used to transmit and/or receive communication signals between a base station and a user equipment. The system equipment can also be located at places other than base stations. The term xe2x80x9cuserxe2x80x9d hereinafter is used interchangeably with the term xe2x80x9csubscriberxe2x80x9d to indicate an entity (i.e., person, facility or a combination thereof) who is allowed access (when possible) to the communication system. Access to the communication system is the ability of a subscriber to make use of the resources (e.g., system equipment, communication channels) of the communication system. The user equipment (e.g., 140 in cell 104) is typically a cellular phone or any other communication equipment that is used by a subscriber of a communication system. For example, the user equipment can be a wireless portable computer or a pager. The system equipment further comprises processing equipment for retrieving information being carried by the communication signals and for implementing procedures based on communication protocols.
A communication protocol is a set of procedures or processes that dictate how communications between users of the communication system is to be initiated, maintained and terminated. The communication protocol also dictates the communications between a user and system equipment. Communication protocols are part of well known and established standards that are followed by operators of communication systems.
Still referring to FIG. 1, each user communicates with a base station via a wireless communication link. For example in cell 104, user 140 communicates with base station 114 via communication link 156. Typically, each wireless communication link comprises several communication channels. For example, for a Code Division Multiple Access (CDMA) wireless system, the communication link comprises an Access channel, a Paging channel and a Traffic channel. The Access channel is a channel through which user equipment transmits protocol information to a base station. The protocol information is information used by the system equipment operate and/or control the communication system. For example, a user requesting access to the communication system is allowed to use the communication system after various protocol information have been exchanged between the base station and the user equipment. In allowing the user to have access to the communication system, the system equipment identifies the user as a subscriber of the system, finds resources (e.g., communication channels, base station equipment) that can be made available to the user and allows the user to use (transmit/receive information) such resources in accordance with the protocol being followed by the communication system.
The Paging channel is a channel through which system equipment (e.g., base station) broadcasts protocol information to users of the communication system. The traffic channel is the channel used by the users to communicate with each other or with the system. The information conveyed through the traffic channel is, for example, voice, data, video, facsimile information or any other information typically conveyed by users of communication systems. The traffic channel consists of two channels: the first channel is called the reverse link through which users transmit information which information is received by the base station (or other system equipment); the second channel is called the forward link through which the base station (or other system equipment) transmits lo information to a user. Each user has a forward link and a reverse link assigned by the communication system. In addition to the other channels discussed above, some CDMA systems also have a pilot channel that is used to assist a user to request and obtain access to the communication system.
The pilot channel is a channel through which the system equipment broadcast a pilot signal that covers a certain area (e.g., cell area) of the communication system. The pilot signal serves as a sort of beacon signal that advertises the existence of the base station for any user who wants access to the communication system. The pilot signal also serves as a timing signal for the user equipment; that is, a pilot signal received by a user equipment (e.g., cell phone) is one of several signals used by the user equipment to synchronize its timing to the timing of the base station. The pilot signal is typically a system defined carrier signal; that is, the pilot signal is typically a signal of a single frequency, fc. The user equipment has the proper hardware to receive and detect a pilot signal of appropriate power. Due to a well known phenomenon called the xe2x80x9cDoppler effect,xe2x80x9d the frequency of the pilot signal received by the user equipment is fcxc2x1fd where fd is called a xe2x80x9cDoppler shift.xe2x80x9d The Doppler shift is the change in the carrier frequency that occurs due to the relative motion of user equipment to the base station equipment (or other system equipment) from which the pilot signal is transmitted. When the user equipment is moving away from the base station, the Doppler shift is subtracted from the carrier frequency. When the user equipment is moving toward the base station, the Doppler shift is added to the carrier frequency. It is also well known that the speed of the user equipment relative to the base station equipment directly affects the value (i.e., the amount of shift) of the Doppler shift.
A user initiates a request for access to the communication system by transmitting a probe signal. The probe signal is typically also a signal of a certain frequency which is also affected by the Doppler effect. The probe has two portions: the first portion is called the preamble which is typically a string of xe2x80x9c0xe2x80x9d bits or a string of xe2x80x9c1xe2x80x9d bits. The second portion of the probe is a message portion containing protocol information. The preamble is the portion of the probe that allows the base station (or other system equipment) to detect the probe. The system equipment decodes the message portion of the probe. Once the probe preamble is detected and the probe message is decoded, the system equipment initiates a certain procedure (in accordance with a protocol being followed by the communication system) to provide access to the user that transmitted the probe signal. Prior to initiating the procedure, the system equipment transmits an xe2x80x9cacknowledgexe2x80x9d (ACK) message to the user equipment indicating to the user equipment that the probe signal has been detected. Once the user equipment receives the ACK message it no longer transmits the probe signal and proceeds as per the protocol to obtain access to the communication system.
In many cases the probe signal is not detected by the system equipment because the power (or amplitude) of the probe signal received by the system equipment is attenuated due to various effects of the communication link. In such cases, the user equipment transmits the probe signal repeatedly until it receives the ACK message. For each repeated transmission of the probe signal, the power of the probe signal is increased by a system defined amount hereinafter referred to as xe2x80x9cxcex94.xe2x80x9d A graph of the power of the probe signal versus time is shown in FIG. 2. Each probe is transmitted after a certain time interval xcfx84+xcfx84ri where xcfx84 is a system defined time interval and xcfx84ri is a time interval of random length for the ith probe signal. Thus, according to the graph of FIG. 1, the first probe has an amplitude of P1, the second probe has an amplitude of P2, the third probe has an amplitude of P3 and so on. In general, the probe power can be expressed by the following equation: (1) Pi=P0+xcex94i where the ith probe has power Pi and the initial probe power is P0. P0 is a system defined value that represents the initial probe power. Typically, the attenuation of the probe signal varies in a random manner. A main cause of random variations in the power or amplitude of a probe signal is due to a well known phenomenon called fading.
Fading generally relates to adverse effects on a signal (received by system equipment or user equipment) due to obstacles (e.g., buildings, towers, and other tall structures) and moving objects located between user equipment and system equipment. Therefore, fading affects the pilot signal, the probe signal as well as any other signal transmitted and/or received by the system and user equipment. Fading is caused by interference between two or more versions of a transmitted signal which are received at slightly different times. The fading phenomenon is manifested as amplitude (or power) variations in signals received by system or user equipment. A probe signal or a pilot (or any other signal) experiences different types of fading depending on the particular topography [physical demographics of the terrain] covered by a communication system. Fading is often graphically depicted as signal amplitude (or power) vs. time as shown in FIG. 3 and FIG. 4. FIG. 3 depicts a type of fading commonly known as xe2x80x9cRaleigh fadingxe2x80x9d and FIG. 4 depicts another type of fading called xe2x80x9cLog Normal fadingxe2x80x9d. Generally, as can be clearly discerned from FIGS. 3 and 4, the variations in a signal""s amplitude (or power) due to Raleigh fading occurs more frequently than the variations due to Log Normal fading.
As discussed earlier, fading is caused due to different versions of a signal being received at slightly different times. The correlation between the different versions of the signal being received varies with the speed of the user equipment relative to the system equipment. Correlation between two signals generally refers to an interdependence between two signals. For example, signals that are relatively highly correlated have relatively similar signal characteristics. Signal characteristics are parameters of a signal that are used to describe a signal. Examples of a signal""s characteristic are amplitude, phase, frequency content, and power. Time correlation describes circumstances where there is an interdependence between different versions of the same signal at different instances of time. Thus, two versions of the same signal which are highly time correlated to each other will have similar phase, amplitude, power level and frequency content. Conversely, two versions of the same signal with low time correlation between them will have very little interdependence; the respective characteristics of such signals tend to have random variations with respect to each other as such signals are not as closely related to each other. It follows therefore that the fading for a relatively highly time correlated signals will have less variations (in amplitude, power, phase, frequency) than the fading for a relatively lower time correlated signals.
Referring to FIG. 5, there is shown the fading (Log Normal fading) of the power of a pilot signal received by user equipment moving at speed v1 relative to the system equipment. FIG. 6 shows the fading of the same pilot signal received by user equipment moving at speed v2 relative to the system equipment where v2 greater than  greater than v1. As explained supra, the fading depicted by FIG. 6 has more variations because of the relatively higher speed of the corresponding user equipment. FIG. 5 shows an example of xe2x80x9chigh correlation fadingxe2x80x9d; FIG. 6 shows an example of low correlation fading.
As discussed earlier, the power transmitted by a probe signal is increased by a specific amount (see equation (1)) until the probe signal is detected by the base station. Whenever, relatively large downward power variations (due to fading, for example) occur in a probe signal received by system equipment, a user equipment has to keep increasing its probe signal power level as per equation (1); in such a case, the likelihood of the probe signal interfering with neighboring base stations increases. For example, referring to FIG. 1, in cell 104 user 136 because of its proximity cell 102 may be transmitting a probe signal (to base station 114) that interferes with base station 110. Also, in such a case, the user equipment has to wait for a relatively long, period of time before detection, if ever, by the base station occurs; this is because several xcex94 amounts have to be added to the probe signal to compensate for the power variations.
Depending on the speed of the user equipment and its position relative to the base station, the probe signal received by the base station may experience a deep fade. In cases of deep fading (i.e., large attenuation of signal for, a certain time period), a high correlation deep fade means that the signal will remain in a deep fade for a relatively long time period. In such a case, an additional amount of power has to be added to the probe signal to compensate for the deep fade thus increasing the probability of detection. A low correlation deep fade means that the signal will remain in a deep fade for a relatively short period of lime. In cases of low correlation deep fades, a relatively small amount of power needs to be added to the probe to compensate for the deep fade. It is thus clear that depending on the type of fading that occurs a proper amount of power has to be added to compensate for such fading. It is also clear that the addition of a fixed xcex94 amount for all circumstances will lead to inefficient use of the power available for the probe.
What is therefore needed is a method of adjusting the power of a probe signal to properly compensate for fading effects on the probe signal and thus increase the likelihood that the adjusted probe signal will be detected by the system equipment. What is also needed is a method in which the power available for the probe signal is used efficiently.
The present invention provides an apparatus for generating an adaptively adjusted probe signal to be transmitted by user equipment of a communication system so as to increase the likelihood that such probe signal will be detected by system equipment when transmitted. The adjustment is based on an analysis of communication signals received from the system equipment and communication system constants. In a preferred embodiment, the power of the probe signal is adaptively adjusted and thus the power available for the probe signal is used in an efficient manner.
The apparatus comprises a first module wherein system constants are stored and a second module configured to derive signal characteristics from communication signals received from the system equipment. The apparatus further comprises a third module coupled to the first, and second modules which third module is configured to calculate an adjustment parameter based on the derived signal characteristics and the system constants and apply said adjustment parameter to a probe signal to be transmitted resulting in a modified probe signal. The apparatus then generates an adaptively adjusted probe signal by selecting from the modified probe signal and a system defined probe signal.
In a preferred embodiment, the first module stores a system decorrelation distance, a system scaling factor and a system defined maximum power probe signal. The second module derives a Doppler shift of a pilot signal received by the user equipment. A first section of the third module calculates an adjustment parameter from the decorrelation distance, a system timing signal and the system scaling factor. The first section applies the adjustment parameter to the power of the current probe signal to be transmitted resulting in a modified probe signal. The modified probe signal and the system defined maximum power probe signal are applied to a second section of the third module which second section generates the adaptively adjusted probe signal by selecting the smaller of the modified probe signal and the system defined maximum power probe signal.